Don't just compare cases by capacity. The differece between case length and overall length (or magazine or action length) will affect seating depth which changes the combustion chamber capacity. The 338 Lapua magnum is a relativley short fat case which gives it more usable capacity than it's slightly larger neighbors when used with long low drag bullets. The 338 Win Mag can also be seated out further than SAAMI spec when used in a "long magnum" magazine. Most of the other magnums are pretty crowded in "long magnum" magazines. Compare complete cartidges at workable seating depths in the action you choose to use. Somtimes the smaller capacity cartridge gives higher performance.

I can give you dozens of loads with the 340 wby, 338 lapua, 338 and 338-300 ultramags all out of 28" barrels. Max loads are all within about a hundred fps with all of these except the lapua. Overall the lapua data is the fastest of these and will get about 150 fps spread over the 340 wby. Some may be faster or slower barrels but I am talking averaging data over many rifles tested. From my experience in a hunting situation you will not notice much difference with any of them. I guess water capacity will put you in the ballpark but like mentioned above different case designs may perform better or worse. The 338-378 wby shown has tremendous water capacity but is only 150 fps faster than the lapua. So from slowest to fastest of all of those your looking at about 300 fps. If anyone wants any of that data I will be glad to look it up and post it here.

I have catalogs of data on these 338's mentioned. I love to shoot all of mine but most of my hunting goes to the low side 340 wby or the high side 338-378 wby. In a hunting situation I notice no difference between the 340 wby and the ultramags so I go with less powder, recoil and muzzle blast. When I want a big gun I pick up my 338-378 wby which seperates itself from the rest of the pack. Honestly, if I had an elk out there at a thousand yards give me any of them because they are all very capable rifles. With the extra velocity of the 338-378 it will give you a little edge in the wind which is the hardest thing to judge. I tried since the 70's trying to find the highest velocity 338. Bottom line there is just not much seperation between them with max loads. So just get any of them, accurize it and you are good for long range big game.

__________________
Predictions are difficult, especially when they involve the future